The photoprotection of keratinous substrates, especially skin, is considered by many to be necessary in order to facilitate protection from sunburn and photo-aging, as well as to decrease the chances of skin cancer development. There are typically two types of UV sunscreens used to accomplish photoprotection, namely, inorganic UV filters and organic UV filters.
Inorganic UV filters such as titanium dioxide and zinc oxide are typically employed in large quantities in order to ensure proper coverage/maximum protection over the surface onto which they are applied. As a result, they have a tendency to feel dry and impart an undesirable white color onto the treated surface.
Organic UV filters are typically classified as being either water-soluble or oil-soluble, based on their solubility. Oil-soluble UV filters, such as homosalate, octocrylene and avobenzone, while easy to incorporate into emulsions, often impart a greasy and tacky feeling onto a user's skin which consumers consider unpleasant and hence, undesirable.
Water-soluble UV filters do not impart this unpleasant feeling as they are considered by consumers to provide a much lighter skin feel. Water-soluble UV filters are often used in the form of a sodium salt (i.e., an electrolyte) in order to improve their solubility profile. The problem, however, is that such filters, due to their electrolytic properties, are difficult to formulate with when it comes to long-term stability. This lack of stability oftentimes manifests itself in the form of re-crystallization of the filters in the composition, causing them to separate from the emulsion. Additionally, water-soluble UV filters tend to have a detrimental effect on any thickener ingredients conventionally found in topical products as they too have a tendency to separate from the emulsion due to the electrolytic properties of the UV filters. As a result, water-soluble UV filters present a challenge for incorporation into emulsions intended for topical application onto a keratinous substrate, as most traditional emulsions are thickened and/or stabilized with natural or synthetic polymers such as gums and polyacrylates, which are very sensitive to electrolytes.
Therefore, it is an object of the present invention to provide a photoprotection composition containing at least one water-soluble UV filter, in a salt or an acid form, having improved stability but without having a greasy and/or tacky feel.